Generally speaking, gas turbine engines, steam turbines and other related hydraulic systems employ phosphate ester fluid lubricants, an example of which is phosphate ester fluid as the primary lubricating material. Although a particularly useful lubricant, the fluid is vulnerable to thermal degradation which results in the generation of acid contaminants in the form of phosphorus and phosphoric acids along with a variety of metal salts from acidic corrosion of internal gas turbine metals.
In an attempt to provide for possible cleaning methods, the prior art has provided filtration of the degraded fluid through Fuller's earth and/or activated alumina for the removal of acids from the thermal degradation. Recently, fluid filtration has progressed to continuous side stream treatment and has employed acid adsorbent medias which include activated alumina for acid removal.
Regarding the activated alumina and Fuller's earth, although these filter media are generally useful in the process of adsorbing acids, they contribute to the contamination level in the fluid and this has a significant impact on fluid quality and therefore operation of the apparatus employing this fluid. In the case of the Fuller's earth, adsorbed acids dissolve free calcium and magnesium which are naturally abundant in the Fuller's earth media. The calcium and magnesium enter the lubricating fluid as a soluble metal-salt and electrolytically plate out on hot engine components such as shafts, bearings and seals. The result is premature component wear and concomitant failure.
Similarly, activated alumina although generally understood to be a better adsorbent, additionally contributes sodium as a metal to the fluid. The problem is particularly pronounced when the sodium level becomes elevated beyond 90 parts per million (p.p.m.). At this level, or greater, the sodium has a tendency to react with the additional fluid degradation products in the lubricant to produce, for example, sodium phosphate and phosphites. Generally speaking, sodium phosphates chemically are commonly known as detergent soaps. The result can produce severe fluid foaming which, in turn, can cause lube oil pump cavitation as well as bearing and seal failures.
In an attempt to satiate the difficulties associated with decontaminating lubricant fluid of this variety, the art has proposed numerous methods, typical of which is indicated in U.S. Pat. No. 4,741,857, issued May 3, 1988 to Horwitz et al. Horwitz et al. teaches a method of purifying neutral organophosphorus extractants which primarily involves the mixing together of CMPO, TBP and NPH. The compounds are mixed together to form an organic extractant that is adapted to pick up the radiolytic and hydrolytic degradation products. In view of the fact that the acids are in both forms, i.e., salt and acid, the method requires both cation and anionic exchanges. The disclosure indicates that the extractants are contacted for at least 30 minutes with agitation. Further, the process is a two step process where the material to be treated must be contacted with the cation exchange in a first step to form a first solution and then subsequently contacted with the anion resin to complete the acid removal.
The above process is clearly limited in that it involves extensive treatment time and cannot produce a substantially contaminant free fluid rapidly and in a single pass through a single ionic exchange material.
Further prior art related to purification of fluids using ionic resins includes U.S. Pat. No. 3,708,508, issued to Schulz. The method is directed to the purification and recovery of tri-n-butylphosphate used in reprocessing nuclear fuel.
In view of what has been previously proposed in the art, it would be desirable to have a more efficient process where a spent or degraded lubrication fluid could be cleansed while in use without removal. This clearly has advantages in terms of reducing the probability of damage from using contaminated lubricant, clear cost savings since the material can be reused, as well as reducing the volume of chemical compounds which have to be handled carefully from an environmental point of view.